Stress of Sliding Plates Builds Near Chile

When a magnitude-8.8 earthquake struck off the coast of Chile last February, geophysicists and seismologists were not surprised. The quake’s epicenter was on a roughly 200-mile stretch of a fault where stresses had been building for nearly two centuries, and experts had expected that one day the strain would be relieved in a cataclysmic event.

But as scientists have pored over volumes of data from what may turn out to be the best-studied major earthquake yet, they have concluded that the ground movement during the quake did not relieve the stresses as anticipated. The greatest seismic slip was outside the 200-mile segment, known as the Darwin gap since Charles Darwin happened to witness the last earthquake along it, in 1835.

“The pattern of slip was quite different from what would have been expected,” said Stefano Lorito, a geophysicist with the National Institute of Geophysics and Vulcanology in Rome. While there was one area of slip south of the epicenter that was within the Darwin gap, he said, the area of greatest movement was north of the gap in an area where an 8.0 earthquake occurred in 1928.